1National Lab for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 2LANSCE Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 3Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029 4Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA 5Department of Physics, Southern University of Science and Technology (SUSTech), Shenzhen 518055 6Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094 7Engineering Laboratory of Specialty Fibers and Nuclear Energy Materials, Ningbo Institute of Industrial Technology, Chinese Academy of Sciences, Ningbo 315201
Abstract：Methane clathrate hydrate (MCH) is a promising energy resource, but controllable extraction of CH$_{4}$ from MCH remains a challenge. Gradually replacing CH$_{4}$ in MCH with CO$_{2}$ is an attractive scheme, as it is cost efficient and mitigates the environmentally harmful effects of CO$_{2}$ by sequestration. However, the practicable implementation of this method has not yet been achieved. In this study, using in situ neutron diffraction, we confirm that CH$_{4}$ in the 5$^{12}6^{2}$ cages of bulk structure-I (sI) MCH can be substituted by gaseous CO$_{2}$ under high pressure and low temperature with a high substitution ratio ($\sim $44%) while conserving the structure of the hydrate framework. First-principles calculations indicate that CO$_{2}$ binds more strongly to the 5$^{12}6^{2}$ cages than methane does, and that the diffusion barrier for CH$_{4}$ is significantly lowered by an intermediate state in which one hydrate cage is doubly occupied by CH$_{4}$ and CO$_{2}$. Therefore, exchange of CO$_{2}$ for CH$_{4}$ in MCH is not only energetically favorable but also kinetically feasible. Experimental and theoretical studies of CH$_{4}$/CO$_{2}$ substitution elucidate a method to harness energy from these combustible ice resources.
2020, Vol. 37 
